1. Field of the Invention
The invention relates to a thermal transfer image receiving sheet which is used with a thermal transfer sheet superposed.
2. Description of the Related Art
A method for forming a full-color image by superposing a thermal transfer sheet and a thermal transfer image receiving sheet each other is known as a method for forming an image using thermal transfer. In the thermal transfer sheet, a sublime dye as a record material is supported on a base material sheet such as paper and a plastic film. In the thermal transfer image receiving sheet, a receiving layer for the sublime dye is formed on one surface of the paper and the plastic film. Since the sublime dye is used as a color material in the method, a density gradation can be freely adjusted in dot units, and the same full-color image as a manuscript can be expressed on the receiving sheet. Since the image formed by the dye is very clear and excellent in transparency, the image also excels in the reproducibility of neutral tints and gradation, and the high-quality image equal to the image of a daguerreotype can be formed.
In a sublime thermal transfer type printer, a dye receiving layer consisting mainly of a dyeable resin with a dye (a resin having the character to be easily dyed with a dye) is preferably formed on the base material of the image receiving sheet so as to form a print image with high image quality on the receiving sheet at high speed. When a paper material such as a coat paper and an art paper are used for the base material of the image receiving sheet, the sensitivity receiving the dye on the receiving layer is lowered since the thermal conductivity of the materials is comparatively high.
Then, a biaxial stretched foaming film which consists mainly of a thermoplastic resin such as polyolefin, is used for a base material of the image receiving sheet and has voids therein can be used. Since the image receiving sheet in which the film is used for the base material has uniform thickness, flexibility, and thermal conductivity being smaller than that of paper or the like consisting of cellulose fiber, there is the advantage that the image having uniform and high density can be obtained. However, when the biaxial stretched film is used for the base material of the image receiving sheet, the residual stress at the time of drawing is eased by heat at the time of printing, and the film is shrunk in a drawing direction. As a result, curl and wrinkle are generated on the image receiving sheet, and trouble such as paper jam may be caused on paper when the image receiving sheet travels in a printer.
There is an example in which a laminate sheet constituted by laminating a biaxial stretched foaming film having voids to a core having a relatively small thermal shrinkage or a core having large elastic modulus is used for the base material of the image receiving sheet so as to improve the drawbacks. A thermal transfer image receiving sheet exists, wherein a non-foaming plastic film is superposed to a core via an adhesive containing a foaming agent, and the adhesive layer has a porous structure by foaming the foaming agent in the state (see Japanese Patent Application Laid-Open No. 6-239040). Also, a technique is known, in which a porous layer coating liquid obtained by mixing hollow particles with a binder resin is coated on a base material sheet, and thereby a porous layer is formed (see Japanese Patent Application Laid-Open No. 2002-212890).
However, since the biaxial stretched film having voids has large retractility, it is difficult to control the tension at the time of laminating, and thereby productivity is lowered. Also, the manufacturing cost is greatly increased. The management of a coating condition requires time when the porous layer coating liquid is coated.
It is an object of the invention to provide a thermal transfer image receiving sheet which can enhance sensitivity due to low heat conductivity and can be easily manufactured at a low cost compared with the lamination of a foaming film and the coating of a coating liquid. It is another object of the invention to provide a method for manufacturing the same.